Tannerella forsythia (Bacteroides forsythus) is a gram-negative oral anaerobe implicated in the development of periodontal disease pathogenesis. T. forsythia remains one of the most understudied periodontal pathogens, partly due to the fastidious growth requirements for culturing this bacterium as well as the fact that genetic manipulation of this organism has only recently been accomplished. Moreover, the pathogenicity of this organism in animal models has only been documented recently. We identified a surface-associated as well as a secreted protein, BspA, in T. forsythia. The BspA protein belongs to the leucine-rich-repeat as well as to the bacterial immunoglobulin-like superfamilies of proteins. Studies utilizing in vitro model systems have shown that the BspA protein induces the release of proinflammatory cytokines/chemokines from host cells by activating toll- like receptor 2, as well as confers bacteria the ability to invade epithelial cells by activating intracellular signaling leading to cytoskeleton changes. In addition, BspA mediates coaggregation of T. forsythia with Treponema denticola and Fusobacterium nucleatum. Studies in a mouse model of bacterially-induced alveolar bone loss showed that a BspA-defective T. forsythia mutant was avirulent, suggesting that BspA is an important virulence factor of T. forsythia. This proposal has following specific aims. Aim 1 is directed toward: characterization of BspA-induced activation of innate responses through toll-like receptor 2 signaling; structure function studies of the BspA protein, and; identification of the cellular receptor including intracellular signaling associated with BspA-mediated bacterial entry into epithelial cells. In addition, regulatory mechanisms of bspA gene expression will be investigated. In aim 2, in vivo role of BspA protein relative to colonization and inflammation will be evaluated in a murine model. Moreover, the immune response to the BspA protein in patients with periodontitis will be determined to address the importance of BspA in pathogenesis. These approaches will be important in determining the roles of the BspA protein in colonization as well as in inflammation. In the long term, understanding the role of the BspA protein in pathogenesis and underlying mechanisms will be vital in developing novel intervention strategies against periodontal disease.Tannerella forsythia is a gram-negative oral anaerobe implicated in the development of periodontal disease pathogenesis and is one of the most understudied periodontal pathogens. This bacterium expresses a cell surface-associated as well as secreted virulence factor, the BspA protein, which has been shown to play important roles in the bacterial pathogenicity. The studies proposed in this application are aimed at understanding the mechanisms of BspA-induced pathogenesis in periodontal disease and will be vital in developing therapeutic strategies against periodontal diseases in future.